Sex Differences in Early Childhood Trajectories of Internalizing and Externalizing Behaviors in Autism: Findings From the Norwegian Mother, Father and Child Cohort Study (MoBa)

PURPOSE: To examine trajectories of parent-reported internalizing and externalizing behaviors at 18 months, 36 months, and 5 years among children with autism spectrum disorder (ASD) compared with children with no recorded diagnosis (NoDx), and to test whether these patterns differed by sex. METHODS: CBCL-derived internalizing and externalizing scores were constructed from harmonized MoBa items available across the three waves. Linear mixed-effects models tested time x diagnosis x sex effects in the ASD-vs-NoDx sample (N = 61,553), with a secondary ASD-only sex analysis. RESULTS: ASD-NoDx differences were not clearly evident at 18 months, but emerged by 36 months and were clearest at 5 years. Internalizing and externalizing showed significant time x diagnosis effects, but no evidence that diagnosis differences varied by sex. In ASD-only models, sex differences were modest; boys showed higher externalizing than girls only at 5 years. CONCLUSION: Broad CBCL-derived emotional and behavioral differences associated with ASD became more detectable across the toddler-to-preschool transition. Sex differences were small relative to diagnosis-related developmental divergence.